1. Field of the Invention
The present invention relates to a semiconductor card called an IC card, for example, and more particularly, to a semiconductor card (which will hereinafter be the referred to as an "information card") having a volatile memory element and a back up battery and a manufacturing method thereof.
2. Discussion of Related Art
In recent years, in the field of semiconductor cards, it has been the procedure to substitute volatile memory for nonvolatile memory for the purpose of speeding up signal processing as well as for the reduction of cost. Use of the volatile memory element naturally requires a backup battery to be built in the card.
An example of this kind of semiconductor card is shown in FIGS. 9 and 10. FIG. 9 is a sectional view of a semiconductor card in a state before being molded with resin, and FIG. 10 is a sectional view of the semiconductor card in a state after being molded with the resin.
As shown in FIG. 9, a thin casing 51 comprising an upper casing 51a and a lower casing 51b is formed of a synthetic resin. Separately from the casing 51, a printed circuit board 54, on which various electronic components 52 each having the function of volatile memory, control section, interface portion or the like, and a backup battery 53 are mounted, is encased by the lower casing 51b.
The upper casing 51a is put on the lower casing 51b so as to define a space 55 therebetween. A law molecular weight liquid epoxy resin is poured into the space 55 and hardened to form a sealing layer 56 (see FIG. 10).
With this contemplated procedure and structure, however, it takes a long time to harden the epoxy resin and the epoxy resin must be subjected to defoaming after being poured into the casing 51, resulting in very poor productivity and hence an increased cost. Moreover, since the epoxy resin has a high coefficient of contraction or shrinkage due to the hardening thereof, a shrinkage cavity is generated within the sealing layer 56 to form a gap between the casing 51 and the sealing layer 56, resulting in a deformation (depression) of the card.
In order to eliminate the above problems, a method for manufacturing an information card or semiconductor card has been developed in which a sealing layer of a thermosetting resin having a high molecular weight is formed by an injection molding process in such a manner that a printed circuit board having various electronic components mounted thereon is embedded therein. In this case, instead of subjecting the backup battery to the injection molding together with the printed circuit board, a recess for receiving the battery is formed at a predetermined position in the sealing layer so that, after the formation of the sealing layer, the battery is introduced into the recess and an opening of the recess is closed by a battery cover.
The reason why the battery is not subjected to the injection molding at the time of forming the sealing layer as described above is to prevent the battery from being thermally effected.
Further, the reason why a thermosetting resin is used to form the sealing layer is that the thermosetting resin is more excellent in general than a thermoplastic resin in terms of water resistance and mechanical strength.
However, since the sealing layer having the recess is formed of the thermosetting resin as mentioned above, it is impossible to provide a sufficient welding strength between the sealing layer and the battery cover also formed of a thermosetting resin. In particular, considering mass productivity, an ultrasonic welding is suitable by which the welding operation can be completed in a short time. However, since both the sealing layer and the battery cover are formed of a thermosetting resin as described above, ultrasonic welding is also not applicable.
The information card or semiconductor card, which is portable judging from its shape and size is required to have a high water resisting property since there are possibilities of the card being suddenly splashed with water, such as by being dropped into water, or being washed or getting wet in the rain.